Question

Meselson and Stahl proved the semi-conservative nature of DNA replication using the isotope ___.

• A. \({}^{32}\text{P}\)
• B. \({}^{35}\text{S}\)
• C. \({}^{14}\text{C}\)
• D. \({}^{15}\text{N}\)

Hint:

The heavy isotope had to be part of every DNA base so that all DNA became heavy.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept
Meselson and Stahl (1958) grew E. coli for many generations in a medium whose only nitrogen source was \( {}^{15}\text{N} \), so the DNA became heavy.

Step 2: The experiment
The cells were shifted to normal \( {}^{14}\text{N} \) medium and the DNA of each generation was spun in a caesium chloride density gradient.

Step 3: Result
After one generation all DNA was hybrid, and after two generations half was hybrid and half light. This proves semi-conservative replication.

Final Answer:
The isotope used was nitrogen-15. \[ \boxed{\text{(D) }{}^{15}\text{N}} \]